1. Field of the Invention
The present invention relates generally to an encoding/decoding apparatus in a mobile communication system, and in particular, to an interleaving apparatus and method for an encoder using a serial concatenated convolutional code (SCCC).
2. Description of the Related Art
In general, a W-CDMA (Wideband Code Division Multiple Access) or CDMA-2000 communication system requires data transmission at a low bit error rate (BER) ranging from 10.sup.-4 to 10.sup.-6, and requires a high data reliability in the situation where a signal-to-noise ratio (SNR) is relatively low in data transmission of a satellite system. To satisfy such requirements, most mobile communication systems encode and decode digital information to be transmitted over a wired or wireless link, using a parallel concatenated convolutional code (PCCC) or a turbo code for forward error correction. The use of PCCCs has improved performance as compared with existing convolutional code, and in particular, has superior performance when the SNR is low. Therefore, the PCCC has been adopted as the Forward Error Correction (FEC) technique for data communication in the IMT-2000 system and is the subject of active research.
Performance of the PCCC is determined according to SISO (Soft-In-Soft-Out) iterative decoding at a receiver, and an encoder using the PCCC (hereinafter, referred to as a PCCC encoder) has a characteristic which is dependent on two parallel-connected component encoders and a turbo interleaver connected between them. The component encoders commonly use a recursive systematic code (RSC). The PCCC has a longer free distance d.sub.free as compared with existing convolutional code.
On the other hand, an error correction code, which is obtained by directly connecting two or more error correction codes, has been typically used in systems which require a very low BER. For example, the provisional UMTS (Universal Mobile Telecommunication System) standard specifies that a serial concatenated convolutional code (SCCC) should be used for a BER of below 10.sup.-7.
FIG. 1 shows a structure of an encoding device that uses a SCCC (hereinafter, referred to as an SCCC encoder). In particular, FIG. 1 shows a 4-state SCCC encoder in which the component encoders have 4 states.
Referring to FIG. 1, the SCCC encoder includes two component encoders 10 and 40, and an interleaver 30 which connects them in series. Performance of the SCCC encoder is determined depending on performance of the component encoders 10 and 40 and the characteristics of the internal interleaver 30. If the component encoders are set as in the PCCC encoder, the total d.sub.free of the SCCC encoder will depend on the characteristics of the internal interleaver 30. As a result, the characteristics of the internal interleaver 30 will determine the whole performance of the SCCC encoder. That is, if the component encoders 10 and 40 are given, the interleaver 30 becomes the unique factor which determines the whole performance of the system.
In FIG. 1, an outer encoder 10 encodes input information data U.sup.o and a multiplexer 50 converts the output of the outer encoder 10 in such a manner that two input bit streams are converted to one serial bit stream. Thereafter, a puncturer 20 receiving the output of the multiplexer 50, generates an outer code C.sup.0 of rate 2/3. The generated outer code C.sup.o is provided to an inner encoder 40 as U.sup.i through the SCCC interleaver 30. The inner encoder 40 of rate 1/2 encodes U.sup.i to generate C.sup.i, and a multiplexer 60 converts C.sup.i in such a manner that two input bit streams are converted to one serial bit stream, and transmits its output to a channel.
Conventionally, only the block interleaver or the PN (Pseudo Noise) interleaver was used as the interleaver of the SCCC encoder, on the assumption that the performance of the SCCC encoder has little dependence on the performance of the interleaver. That is, it was assumed that the performance would be guaranteed, if the distance and randomization of the preceding code are guaranteed for the following encoder. However, in order to optimize performance of the SCCC encoder comprised of two cascaded encoders, both the random property and the distance property should have been considered. But, such properties were conventionally overlooked. In other words, the fact was overlooked that, if the coded symbol distance was maintained while transmitting random code symbols, then consecutively generated errors could be fully corrected at the outer decoder.
Therefore, in order to optimize performance of an SCCC encoder, it is necessary to design an SCCC interleaver which satisfies both the distance property and the random property.